(1) Field of the Invention
The present invention relates to a fuel vapor processing apparatus and method of an internal combustion engine, and particularly to a fuel vapor processing apparatus and method where the combustion method is switched between homogeneous combustion and stratified combustion according to operating conditions of the engine.
(2) Related Art of the Invention
The conventional fuel vapor processing apparatus of an internal combustion engine for vehicles are equipped with a canister for adsorbing the fuel vapor generated in a fuel tank, and a purge control valve mounted in a purge passage extending from the canister to an intake system of the engine for controlling the purging quantity of the fuel vapor (refer to Japanese Unexamined Patent Publication No. 7-42588).
On the other hand, a direct injection-spark ignition internal combustion engine where fuel is directly injected to the combustion chamber is being widely noticed recently. In this type of engine, it is common to switchingly control the combustion method in accordance with the operating conditions of the engine. The combustion method is switchingly controlled between a homogeneous combustion where fuel is injected during the intake stroke so as to diffuse the fuel in the combustion chamber to form a homogeneous mixture, and a stratified combustion where fuel is injected during the compression stroke so as to form a stratified mixture centralizing around the ignition plug (refer to Japanese Unexamined Patent Publication No. 59-37236).
When the combustion method is switchingly controlled, if purging of the fuel vapor is performed during the stratified combustion in the same manner as during the homogeneous combustion, the introduction of the fuel vapor generates a very rich air-fuel ratio condition for stratified combustion, since the target air-fuel ratio during the stratified combustion is set to a very lean level. This exerts undesirable influence on the operating performance and exhaust air. Therefore, during the stratified combustion, the purging of the fuel vapor is either reduced or prohibited.
However, when the stratified combustion is continuously performed, if the purging of the fuel vapor is reduced or prohibited during the stratified combustion mode, the canister may overflow and the vapor may be discharged to the atmosphere. In order to prevent such overflow, there is a need to increase the capacity of the canister, which leads to increase of the manufacturing cost.
Therefore, during stratified combustion, the combustion method is forced to be switched temporarily to the homogeneous combustion by a predetermined cycle, so as to perform the purging of the fuel vapor. The cycle is set in accordance with the vehicle speed, which is set to be shorter as the speed increases.
However, the trend in the vehicle speed and the generation quantity of the fuel vapor may vary, and there are cases where the stratified combustion may not be switched to the homogeneous combustion by the best cycle. In some cases, the forced switching cycle is too long, which leads to overflow of the canister and discharge of the vapor to the atmosphere, or too short in other cases, which leads to deterioration of the fuel economy.
Accordingly, the present invention aims at switching the stratified combustion to the homogeneous combustion by the best cycle, so as to prevent the overflow of the canister, and at the same time, improve fuel economy.